Examples of a water electrolysis method include “solid polymer water electrolysis” in which an anode catalyst and a cathode catalyst are coated on a solid polymer electrolyte, “high-temperature steam electrolysis” in which electrolysis is performed at high temperature and with high-temperature steam using a solid electrolyte of an oxygen ion conductor, and “alkaline water electrolysis” which uses a water solution of KOH or a water solution of NaOH as an electrolyte.
The “solid polymer water electrolysis” uses a fluorine-containing perfluoro sulfonic polymer as an electrolyte with a platinum-containing catalyst coated on opposite sides of the fluorine-containing perfluoro sulfonic polymer as a hydrogen generating electrode and an oxygen generating electrode. This method involves a high current density and low power consumption and is thus widely used. An electrolytic cell is formed by contact of a solid material such as a power feeder/electrode/solid polymer electrolyte. Thus, the method has the disadvantage of having difficulty producing a large electrolytic cell. Furthermore, the solid polymer electrolyte is highly acidic, and thus, a platinum-containing material needs to be used as a catalyst, and an expensive corrosion-resistant material needs to be used as a power feeder.
The “high-temperature steam electrolysis” is a method of electrolyzing steam at approximately 800 to 1000° C. using a solid electrolyte of an oxygen ion conductor such as yttria-stabilized zirconia. This method uses the high temperature and thus, achieves high energy efficiency for water electrolysis. However, the electrolytic cell is mainly composed of a ceramic material, and thus, the method has a serious technical problem associated with high temperature operations and has not been put to practical use.
The “alkaline water electrolysis” is a method already used for 80 years or more, and large water electrolysis plants (3300 Nm3/h-hydrogen) have been constructed. However, the method involves high energy consumption and thus, achieves only an approximately five to ten times lower current density than the “solid polymer water electrolysis”. The method also still needs high electrolytic cell construction costs.
In recent years, attention has been paid to problems such as global warming and a decrease in environments and underground resources. As a solution for these problems, hydrogen has gathered attention as renewable energy or clean energy. However, such renewable energy is not only locally biased but also involves a significant fluctuation in output. Thus, transmission of power generated from natural energy to general power systems has limitations. Another problem is that, for example, surplus power is generated as a result of a significant change in the amount of generated power depending on weather or season. Thus, manufacture, storage, and transportation of hydrogen based on water electrolysis are now gaining attention. That is, attention is being paid to the manufacture of inexpensive storable hydrogen using inexpensive surplus power, the transportation of the hydrogen as needed, and the utilization of the hydrogen as clean energy source or material.
As a method for manufacturing hydrogen by electrolysis, an alkaline water electrolysis apparatus is expected to be used as a large-scale hydrogen manufacturing apparatus because, for example, the apparatus needs less expensive facility costs than other water electrolysis apparatuses and has already been successfully used in commercial plants. However, many problems need to be solved before an alkaline water electrolysis apparatus needing only low facility costs and providing high performance compared to conventional techniques is implemented using power such as renewable energy which fluctuates sharply in voltage or current within a short time.
In alkaline water electrolysis, a reduction in energy loss is important to improve performance. Examples of the energy loss include the overvoltage of the cathode, the overvoltage of the anode, an Ohmic loss in a partition wall, an Ohmic loss associated with the electrolytic solution, and an Ohmic loss resulting from the structural resistance of the electrolytic cell. If these losses can be reduced and the electrolysis current density can be increased by improving the structure of the electrolysis unit, the size of the facility can also be reduced, thus also enabling a substantial reduction in construction costs. However, the current density of the alkaline water electrolysis is low and 1340 A/m2 to 2000 A/m2 as illustrated in “Non-Patent Literature 2”. Thus, a large number of electrolytic cells are provided, the facility is large in size, and the facility costs are high. Furthermore, bipolar plates are used, and thus, when electrolysis is performed using an unstable power supply with a significant fluctuation in load, for example, outgassing may be improper, and moreover, the electrodes may be degraded.
For the structure of the electrolysis unit, a structure is desired which is simple and involves low manufacture costs and low voltage loss. Such an electrolytic cell as illustrated in Non-Patent Literature 1 and Patent Literature 2 has a structure which uses no bipolar electrolysis unit and in which an electrode plate, a gasket, and a partition wall frame are separately provided and in which the components are stacked so as to provide an electrolytic cell. Thus, the electrolytic cell has the disadvantage of being difficult to assemble and maintain. Furthermore, a long distance is present between the anode and the cathode, and thus, another problem of the electrolysis unit is a high Ohmic loss associated with an electrolytic solution.
Patent Literature 2 discloses a zero gap structure which has a continuous, rectangular uneven cross section and which is joined to the electrodes by brazing. The structure allows a small electrolytic cell to be produced, but for a larger electrolytic cell, needs a large facility that joins the electrodes and the rectangular uneven surface together and achieves poor productivity. Moreover, for a large electrolytic cell, the structure may involve a large amount of gas bubbles retained in an internal upper position of the cell and is thus not preferable.
Patent Literature 3 discloses a zero gap electrolysis unit for chlor-alkali electrolysis. However, the electrolysis unit comprises a gas liquid separation chamber, and an anode chamber and a cathode chamber are large in depth, and thus, when an electrolytic cell is formed by arranging a large number of the electrolysis units, the resultant electrolytic cell is long. The zero gap electrolysis unit also has the disadvantages of being heavy and expensive and needing a large installation area for the electrolytic cell. Furthermore, as materials that allow chlorine and hydrogen to be generated, titanium is used for the anode chamber, and nickel is used for the cathode chamber. Thus, to produce a bipolar electrolysis unit, the zero gap electrolysis unit needs to join a nickel partition wall and a titanium partition wall together back to back and is complicated.
Patent Literature 4 discloses a nonconductive frame in which a cell structure includes a peripheral portion with channels supporting the anode and the cathode, respectively, and an end plate to which the anode and the cathode are attached. This electrochemical cell structure comprises the components including the end plate and integrated together using an adhesive, polymer welding, or the like. The cell structure enables a small electrolysis apparatus to be produced but has much difficulty producing a large electrolytic cell that generates a large amount of hydrogen.
For alkaline water electrolysis, needs for an anode that generates oxygen include, besides a low overvoltage for oxygen generation, the unlikelihood of corrosion of a base material for the electrodes or a catalytic layer, dissolution into the electrolytic solution, and the like in an environment in which exposure to alkaline water containing NaOH, KOH, or the like occurs, in spite of the use of an unstable current such as renewable energy. Thus, as a conductive base material for the anode, nickel, nickel alloy, stainless steel, or iron or stainless steel with nickel-plated surfaces is used. Furthermore, as an electrode catalyst (catalytic layer), rare metal such as ruthenium or platinum, an oxide mainly comprising porous nickel, nickel, or cobalt, or the like is used.
As a method for reducing the oxygen overvoltage in alkaline water electrolysis using nickel as an electrode catalyst, increasing the surface area of the electrode catalyst to reduce the actual current density has been proposed. For the increased surface of the electrode catalyst, the use of Raney nickel electrodes as a raw material with a large specific surface area has been proposed. The Raney nickel is nickel left after dissolution and removal only of aluminum from a Raney alloy comprising nickel and the aluminum by means of alkali such as NaOH. The Raney nickel has a large number of pores formed therein when the aluminum is dissolved, and thus has porosity and a very large specific surface area. Accordingly, the Raney nickel is very responsive. As a method for producing Raney nickel electrodes, a method is known which involves forming a Raney alloy layer on a surface of conductive base material such as a nickel net by a method such as electroplating or plasma spraying, and then developing the Raney alloy layer by means of alkali such as NaOH (see Patent Literatures 5 to 7 and Non-Patent Literature 3 described below). However, all of the methods pose a problem in terms of performance such as a rise in oxygen generating overvoltage in a short period of time.
Patent Literature 8 discloses electrodes produced by plasma spraying coating, which are not suitable as water electrolysis electrodes in terms of specific surface area, pore size, composition, and the like. Thus, the electrodes as disclosed in Patent Literature 8 fail to provide satisfactory performance as oxygen generating electrodes.